Kinds of Cesium Clocks

Kinds of Cesium Clocks

Cesium clocks are of two general kinds: a "laboratory (or primary) standard" about as large as a railroad flatcar and a "commercial (or secondary) standard" about as large as a suitcase. Only a few laboratory standards exist; they are used at research labs for frequency measurements of the highest accuracy. Examples are the NIST-7 standard at the National Institute of Standards and Technology (NIST) in Boulder, CO and the atomic fountains at NIST, Physikalisch-Technische Bundesanstalt (PTB), Germany, the Paris Observatory, France, and USNO. Commercial standards, being industrially produced, are cheaper, but still provide state-of-the-art measurement of precise time and time interval. A timing center maintaining an ensemble of such clocks can average their readings to produce a "mean timescale" for scientific and public use.
The U.S. Naval Observatory operates about 70 such cesium clocks, as well as other precision clocks like hydrogen masers, in 18 vaults whose temperature and, usually, humidity are closely controlled in order to minimize perturbations by their environment. The time measurements are made by devices called time-interval counters that compare each clock's time against that of one "Master Clock," whose frequency is steered to match its time to the average of the other clocks. This time is the Observatory's measure of the atomic time called Coordinated Universal Time (UTC). Some cesium clocks are transported to remote locations in order to synchronize other clocks.